In laser printers, both the engine electro-photographic (EP) process control tables and the formatter pulse-width (PW) control tables are based on print quality characterization done during life testing. However, factors such as the usage rate, EP process components, and printing environment will vary over time. It is not possible to anticipate how these factors will vary from life test conditions. Changes in these factors cause noticeable and typically objectionable color density differences between printers over time.
These color differences are a significant cause of service calls on color laser printers, which usually result in replacement of the drum cartridge and/or replacement of the color developer. Prior methods of correcting the color differences without a service call include installing optical density detectors inside the printer. The optical density detectors provide feedback to the printer which, in turn, adjusts the color density levels. This approach adds cost and complexity to the printer. Another approach is to print out a test page and to compare the test page with a reference page, typically included in the user's manual. This approach is inconvenient, because it requires the user to keep track of the reference page, and difficult, because it requires from the user the ability to accurately compare the test page with the reference page.
In order to maintain consistent color densities in offset printing, a dot gain test is used. In the color printing industry, offset printing is the standard for producing high quality, color results. A dot gain test compares the appearance of fine dots with the appearance of much coarser dots. The high perimeter/area ratio of the fine dots makes them much more sensitive to factors that cause more or less ink to be deposited. A gradient of fine dot densities is printed in a uniform coarse dot background. When viewed from a distance where the coarse dots cannot be independently distinguished, the mid-spectrum area of the fine dot gradient blends into the coarse dot background, if the color density is correct.
Accordingly, given the forgoing backgrounds relating to offset printing, color laser printers, and color density levels, objects of the present invention are to provide a new test page, system, and method for setting the color density level of a laser printer.